Add Galician as an available language

What does this MR do?

Add Galician as a language option:

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/43232

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] Tests added for this feature/bug
• [-] Conforms to the code review guidelines
• [-] Conforms to the merge request performance guidelines
• [-] Conforms to the style guides
• [-] Conforms to the database guides